P0420 on a 2017 Chevrolet Malibu Eco

P0420 means the downstream oxygen sensor pattern suggests the bank 1 catalytic converter is not storing oxygen properly.

Not your vehicle? See general P0420 info.

Most common causes on the 2017 Chevrolet Malibu Eco

Ranked from 687 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Long-term misfire/piston damage accelerating catalyst wear
High severity
This vehicle has documented cylinder misfires from a known piston-pin wear issue and a blown spark plug; repeated unburned fuel reaching the converter from such misfires overheats and degrades the catalyst substrate over time.
Reported in 2 NHTSA complaints
Parts:catalytic converter
2
Failing or incorrect downstream oxygen sensor
Medium severity
One owner's vehicle went through repeated oxygen sensor replacements (including a non-OEM sensor that didn't resolve the issue, then an OEM replacement) as part of chasing a recurring stall/CEL problem, showing this sensor is a recurring weak point on this vehicle.
Reported in 1 NHTSA complaint
Parts:oxygen sensor
3
Worn catalytic converter
High severity
Normal thermal/chemical aging reduces the catalyst's oxygen storage capacity below the efficiency threshold the PCM monitors.
Common cause
Parts:catalytic converter
4
Exhaust leak ahead of the downstream sensor
Medium severity
A leaking manifold, gasket, or joint upstream of the rear sensor dilutes the exhaust sample and mimics a worn catalyst.
Common cause
Parts:exhaust gasket
5
Oil or coolant contamination of the catalyst
Medium severity
Oil or coolant reaching the exhaust stream coats and poisons the catalyst substrate, reducing its measured efficiency.
Common cause
Parts:catalytic converter
6
Physical damage to the catalytic converter
Medium severity
Road debris or impact damage to the underbody-mounted converter can crush or dislodge the substrate, dropping measured efficiency below threshold.
Common cause
Parts:catalytic converter

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These causes are aggregated from NHTSA owner complaints and general repair knowledge — this is not a diagnosis. P0420 on your car still needs shop confirmation before any repair.

Common symptoms

Check engine light
Usually no drivability issue
Failed emissions test
Sulfur smell in some cases
Reduced power if converter is clogged

What to check before replacing parts

Check for misfire and fuel trim codes first

Look for: Any active or recent misfire, or lean/rich trim codes, means raw fuel or an off mixture has likely already damaged or is currently overworking the converter — fix those before touching the catalyst.

Inspect for exhaust leaks ahead of the converter

Look for: A leak before the converter (cracked manifold, loose flange, blown gasket) lets in outside air that confuses the downstream O2 sensor into reading as if the catalyst isn't working, even when it is.

Compare upstream and downstream O2 sensor activity

Look for: A healthy converter makes the downstream sensor read flat and steady while the upstream sensor swings actively. If the downstream sensor switches nearly as fast as the upstream one, the converter isn't storing oxygen properly and is likely worn out.

Check converter temperature and performance

Look for: A converter outlet noticeably cooler than expected relative to its inlet under load suggests little catalytic reaction is happening inside — supports a genuinely failed converter rather than a false reading.

Hold off on replacing the converter until root causes are ruled out

Look for: If misfires, exhaust leaks, or fuel trim issues caused this code, a new converter will fail again quickly — confirm those are clean first.

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Matching symptoms

Check Engine Light